1. Field of the Invention
This invention relates to miniature electrical relays and methods of making same using micromachining techniques.
2. Description of Related Art
Electromechanical relays are switching devices typically used to control high power devices. Such relays generally comprise two primary components--a movable conductive cantilever beam and an electromagnetic coil. When activated, the electromagnetic coil exerts a magnetic force on the beam in the same way that a magnet will pick up a nail. This causes the beam to be pulled toward the coil, down onto an electrical contact, closing the relay. In one type of structure, the beam itself acts as the second contact and a wire, passing current through the device. In a second type of structure, the beam spans two contacts, passing current only through a small portion of itself.
The strength of the magnetic force produced by the coil is a function of the material used in the device, the number of turns in the coil itself, and the amount of current passing through the coil. In a typical device, a large number of turns is used so that the current drawn by the coil is much less than the current switched by the relay.
Designed as "ideal" switches, relays are treated as short-circuits when closed and as open-circuits when open. Typical "ON" resistances are 0.5 .OMEGA. or less. When open, the switches are a physical break in the circuit, providing very high "OFF" resistance on the order of 10 M.OMEGA. or more. Because a relay is a closeable break in the wiring of a circuit, there are very few constraints as to how or where they can be used in a circuit. In contrast, circuits using solid state switches such as power transistors and MOSFETs must be designed to allow one of the terminals of the switch to be connected to one of the power rails. The "ON" and "OFF" resistances of such devices also tend to be worse by an order of magnitude or more than those of electromagnetic relays. Further, solid state relays often require large, expensive heat sinks when passing high current loads, a limitation eliminated by electromechanical relays.
Solid state relays and power transistors are small, thus allowing them to be used where space is at a premium. Micro electromechanical relays (microrelays) have been proposed as an alternative to power electronics with most of the benefits of conventional electromechanical relays but sized to fit the needs of modem electronic systems. See, for example, Hosaka et al., Electromagnetic Microrelays: Concepts and Fundamental Characteristics, IEEE 0-7803-0957-/93 (1993), and references cited therein.
However, prior microrelays are overly complex and difficult to manufacture. Accordingly, the present inventors have recognized that there is a need for improved designs and manufacturing techniques for microrelays.